Knex.js multiple chained queries - node.js

Question

I'm currently working on a project in express and I'm using knex.js to handle migrations and queries.
I'm still trying to grasp the concept of promises and how I can run multiple queries with knex.
I have the following code which inserts a new record into my database, this is located in my Unit model file.
this.addUnit = function(unit_prefixV, unit_nameV, unit_descriptionV, profile_id) {
return new Promise(function(resolve, reject) {
knex.insert({ unit_prefix: unit_prefixV, unit_name: unit_nameV, unit_description: unit_descriptionV })
.into('units').then(function(unit) {
resolve(unit)
}).catch(function(error) {
reject(error)
})
})
}
In my routes.js file I then call this on a post request, like so:
app.post('/dashboard/unit/add', ensureAuthenticated, function(req, res) {
let postErrors = []
if (req.body.unit_name.trim() == "") {
postErrors.push('Unit name cannot be empty.')
}
if (req.body.unit_prefix.trim() == "") {
postErrors.push('Unit prefix cannot be empty.')
}
if (req.body.unit_description.trim() == "") {
postErrors.push('Unit description cannot be empty.')
}
if (postErrors.length > 0) {
res.render('addUnit', { errors: postErrors, user: req.user })
} else {
unitModel.addUnit(req.body.unit_prefix.trim(), req.body.unit_name.trim(), req.body.unit_description.trim(), req.session.passport.user.id).then(function(unit) {
res.redirect('/dashboard')
})
}
})
This successfully inserts a new record into my units table, however, I would like to select the user id from the users table with the matching profile_id and then insert another record into my users_units table. All within the this.addUnit function.
For reference my users table consists of:
id
google_id
my users_units table consists of:
user_id
unit_id
I've made an attempt to chain the queries but it only executed the initial insert query and not the others. Here is that rather ugly attempt:
this.addUnit = function(unit_prefixV, unit_nameV, unit_descriptionV, profile_id) {
return new Promise(function(resolve, reject) {
knex.insert({ unit_prefix: unit_prefixV, unit_name: unit_nameV, unit_description: unit_descriptionV })
.into('units').then(function(unit) {
knex('users').where({ "google_id": profile_id }).select('id').then(function(uid) {
knex.insert({ user_id: uid, unit_id: unit }).into('users_units').then(function(user_units) {
resolve(user_unit)
}).catch(function(error) {
reject(error)
})
resolve(uid)
})
console.log(unit)
resolve(unit)
}).catch(function(error) {
reject(error)
})
})
}
Any help will be greatly appreciated!

Answer 1

You're nearly there. There are just a few simple point to grasp :
A Promise can be reolved only once
An explicit Promise is not needed anyway because a naturally occurring promise can be returned
return a Promise at each stage ...
... until the innermost stage, from which the returned value is the finally delivered result.
Errors needn't be eplicitly handled unless you want to inject your own custom error messages or take remedial action.
Having taken all that on board, you might write :
this.addUnit = function(unit_prefixV, unit_nameV, unit_descriptionV, profile_id) {
return knex.insert({ 'unit_prefix':unit_prefixV, 'unit_name':unit_nameV, 'unit_description':unit_descriptionV }).into('units')
// ^^^^^^
.then(function(unit) {
return knex('users').where({ 'google_id':profile_id }).select('id')
// ^^^^^^
.then(function(uid) {
return knex.insert({ 'unit_id':unit, 'user_id':uid }).into('users_units')
// ^^^^^^
.then(function(user_units) {
return { 'unit_id':unit, 'user_id':uid, 'user_units':user_units };
// ^^^^^^
});
});
});
}
If the caller is interested only in success/failure of the process and not the full { unit, uid, user_units } object, then the innermost .then() can be omitted :
this.addUnit = function(unit_prefixV, unit_nameV, unit_descriptionV, profile_id) {
return knex.insert({ 'unit_prefix':unit_prefixV, 'unit_name':unit_nameV, 'unit_description':unit_descriptionV }).into('units')
.then(function(unit) {
return knex('users').where({ 'google_id':profile_id }).select('id')
.then(function(uid) {
return knex.insert({ 'unit_id':unit, 'user_id':uid }).into('users_units');
});
});
}
The promise returned by .addUnit() will still deliver user_units, which the caller can use or ignore.
There's a major proviso to these solutions (and others); a multi-stage update query like this should really be wrapped in a transaction - ie something that allows earlier stages to be rolled back. Otherwise a failure part way through is likely to leave the database in some indeterminate state. This answer is as good a starting point as any.